The invention relates to a charge air and a charge air operation method.
During engine operation water can condense in the charge air system and the inlet manifold and power cylinders of an engine when the intake air temperature drops below the dew-point temperature. The dew point temperature can be defined as a temperature at which a gas would reach saturation for a given boost pressure and ambient humidity conditions.
Modern vehicles are commonly employing exhaust gas recirculation in which exhaust gases are cooled and recirculated back into the combustion chamber to achieve lower chemical emission values. The U.S. Pat. No. 7,007,680 B2 discloses elimination of condensation build-up in the intake manifold and power cylinders by providing a bypass of a charge-air cooler (CAC) and/or an exhaust-gas-recirculation cooler (EGR) juxtaposed to the engine's intake manifold and thus controlling the intake manifold temperature to avoid condensation in the intake manifold. This prevents the formation of acids which could cause corrosion in the intake manifold and power cylinders when condensed water comes into contact with the recirculated exhaust gas.
It is desirable to provide a cooling system encompassing a dual stage turbocharger or mechanical compressor which has improved operationally reliability and durability it is also desirable to provide an operation method of the cooling system.
According to an aspect of the present invention, a charge air system for a combustion engine is proposed, comprising a first boosting system, e.g. a turbocharger stage, for compression of intake air supplied to the engine from a first pressure to a second pressure; a second turbocharger stage for compression of the compressed air to a third pressure; a first heat exchanger being arranged between the first and the second turbocharger stage for cooling the compressed air; a first intake air bypass for modulating the flow of the air, e.g. by at least partially bypassing or, preferably, by fully bypassing the first heat exchanger and/or a first mass flow control unit for controlling the flow of a cooling medium supplied to the first heat exchanger. Favourably, the temperature of the medium being discharged from the first heat exchanger can be kept well above the dew point temperature. Thus, condensation of water and a probable subsequent formation of ice particles can be avoided. As a high-pressure boost device, the second turbocharger stage is especially vulnerable to damage. Particularly during warm-up, low load, low ambient temperature damaging of the air compressor, particularly its compressor wheels, downstream of the first air heat exchanger by water droplets or ice particles breaking away from the first air heat exchanger can be prevented. This can be achieved by entire or partially (modulated) bypass of the air heat exchanger, one or more exhaust recirculation heat exchangers or cooling media and eventually combinations of modulating the amount of air in the bypass and the mass flow of the heat exchanger cooling medium.
In a favourable embodiment, a second heat exchanger can be provided downstream of the second turbocharger stage between the second turbocharger stage and an intake manifold of the engine. A second intake air bypass is provided for modulating the flow of the air through the second heat exchanger, e.g. by at least partially, preferably by fully bypassing the second heat exchanger. Additionally or alternatively, a second mass flow control unit can be provided for controlling the flow of a cooling medium supplied to the second heat exchanger. The first air heat exchanger is the low-pressure air cooler and the second heat exchanger is the high-pressure air cooler for the combustion oxidant, which can be ambient air or an air-exhaust mixture, if exhaust gas is recirculated to the intake manifold. Condensation of water downstream of the second turbocharger stage can be circumvented, preventing the engine intake manifold from damage such as corrosion. Downstream the second air heat exchanger, where the air is at the highest pressure, the risk of condensation of water is even higher than at the first heat exchanger stage as the temperature of the air or the air-exhaust mixture is roughly the same as behind the first air heat exchanger but the pressure is higher and therefore more probably condensation could occur. However, the compressor wheel is more vulnerable to damage as the intake manifold is for corrosion.
According to a further embodiment, an exhaust gas recirculation (EGR) system can provided for recirculating exhaust gas from an exhaust manifold to the intake manifold of the engine. Preferably, a first exhaust heat exchanger can be provided in an exhaust line downstream of the engine for cooling the exhaust gas to a first exhaust temperature. A first exhaust bypass can be provided for modulating the flow of the exhaust through the first exhaust heat exchanger by at least partially or fully bypassing the first exhaust heat exchanger. Additionally or alternatively, a third mass flow control unit can be furnished for controlling the flow of a cooling medium supplied to the first exhaust heat exchanger. A risk for condensation is also present in the one or more exhaust heat exchangers, particularly if it is divided and cooled by air or a low temperature coolant circuit.
If a second exhaust heat exchanger can be arranged in the exhaust line downstream of the engine for cooling the exhaust gas received from the first exhaust heat exchanger to a second exhaust temperature, corrosion of the intake manifold and condensation in the intake air heat exchanger, respectively, in a low pressure EGR system as well as in a high pressure EGR system can be circumvented. For this, a second exhaust bypass for modulating the flow of the exhaust by at least partially or fully bypassing the second exhaust heat exchanger and/or a fourth mass flow control unit for controlling the flow of a cooling medium supplied to the second exhaust heat exchanger can be provided.
For easy controlling the establishment of the desired temperature of the intake air or an air-exhaust mixture supplied for combustion, a control unit can be provided for controlling the amount of air which bypasses at least one of the first heat exchanger, the second heat exchanger, and/or the amount of exhaust gas which bypasses at least one of the first exhaust heat exchanger, the second exhaust heat exchanger; and/or the mass flow of a cooling medium through at least one of the first heat exchanger, second heat exchanger, first exhaust heat exchanger, second exhaust heat exchanger.
Favourably, a sensor unit can be furnished at a discharge port of the first heat exchanger for estimating the dew point temperature of the air discharged from the heat exchanger and/or a sensor unit can be provided at a discharge port of the second exhaust heat exchanger for estimating the dew point temperature of the exhaust discharged from the heat exchanger.
According to another aspect of the invention an operation method for a charge air system is proposed, comprising the steps of compressing intake air from a first pressure to a second pressure in a first turbocharger stage, compressing the compressed air to a third pressure in a second turbocharger stage, estimating the amount of water in air taken in by the first turbocharger stage, estimating a first dew-point temperature of the air discharged by a first heat exchanger arranged between the first and the second turbocharger stage, comparing the first dew-point temperature to an estimated temperature of the air exiting the first heat exchanger, activating an air bypass and/or a cooling-medium mass flow control unit for raising the second temperature above the first dew-point temperature if the second temperature is below the first dew-point temperature. The water in the air taken in by the first turbocharger stage would condense in or after the first air heat exchanger if the charge air temperature drops below the dew-point temperature for a given boost pressure and ambient condition. For a dual stage turbocharging water droplets and even ice particles can occur and damage the compressor wheel or other components of the boost element on the high pressure second turbocharger stage downstream of the first turbocharger stage. For a single stage turbocharger this risk is also present but the consequences are minor compared to damaging a subsequent turbocharger stage.
Condensation of water upstream the boost device which is the second turbocharger stage can be reliably avoided.
Preferably, an air flow through the first and/or a second heat exchanger can be modulated. This can be done e.g. by at least partially bypassing the first and/or second heat exchangers or, preferably, by fully bypassing the first and/or second heat exchangers
Estimating the amount of water in the air taken in can be based on an ambient first temperature, an ambient humidity and the first pressure.
Condensation of water can be avoided further by estimating a second dew-point temperature of the air discharged by the second heat exchanger, comparing the dew-point temperature to an estimated third temperature of the air discharged by the second heat exchanger and activating an air bypass and/or a cooling-medium mass flow control unit for raising the third temperature above the dew-point temperature if the second temperature is below the second dew-point temperature.
Favourably, a delay function can be applied for compensating a warm-up behaviour of the first heat exchanger and/or the second heat exchanger at a cold start of the engine
For dew-point temperature determination, the amount of water in a recirculated exhaust gas downstream a second exhaust heat exchanger can be estimated.
By estimating a fourth dew-point temperature depending on the amount of water in the exhaust gas fed back directly or indirectly to the intake manifold of the engine, comparing the third dew-point temperature to an estimated temperature of the exhaust discharged from the second exhaust heat exchanger, and activating an exhaust bypass and/or a cooling-medium mass flow control unit for raising the temperature above the fourth dew-point temperature if the estimated temperature is below the fourth dew-point temperature, the occurrence of water droplets can be prevented in the recirculated exhaust directly in the intake manifold (high pressure EGR system) or in the intake air upstream of the first turbocharger stage (low pressure EGR system).
In a preferred low pressure EGR system by providing an air-exhaust mixture to the first turbocharger stage, it is preferred estimating the amount of water in the air-exhaust mixture, estimating the first dew-point temperature upstream the first heat exchanger, comparing the first dew-point temperature to the estimated temperature of the air-exhaust mixture discharged by the first heat exchanger, activating the air bypass and/or the cooling-medium mass flow control unit for raising the second temperature above the first dew-point temperature if the second temperature is below the first dew-point temperature.
Favourably, estimating the amount of water in the recirculated exhaust gas can be derived from an air/fuel ratio of the engine.
In a preferred step the second temperature can be measured with a sensor unit. The number of sensor units can be reduced if the second temperature can be calculated based on the first temperature, the first pressure and second pressure and a compressor isentropic entropy derived from the first turbocharger stage.
It is also possible to calculate pressures downstream the first and/or second air heat exchanger based on a rotational speed of the turbocharger stage instead of using sensors. If a variable geometry turbine (VGT) is used, the vane/nozzle position is also provided. If a waste gate turbine (WG) is used, the waste gate position is also provided.
A delay function can be applied for compensating a warm-up behaviour of the first exhaust heat exchanger and/or a second exhaust heat exchanger at a cold start of the engine.
In the drawings, like elements are referred to with equal reference numerals. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. Moreover, the drawings are intended to depict only typical embodiments of the invention and therefore should not be considered as limiting the scope of the invention.